crimson/osd: document fsm of crimson osd

Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/doc/dev/crimson/osd.rst b/doc/dev/crimson/osd.rst
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+osd
+===
+
+.. graphviz::
+
+   digraph osd {
+     node [shape = doublecircle]; "start" "end";
+     node [shape = circle];
+     start -> preboot;
+     waiting_for_healthy [label = "waiting\nfor\nhealthy"];
+     waiting_for_healthy -> waiting_for_healthy [label = "tick"];
+     waiting_for_healthy -> preboot [label = "i am healthy!"];
+     preboot -> booting [label = "send(MOSDBoot)"];
+     booting -> active [label = "recv(osdmap<up>)"];
+     active -> prestop [label = "stop()"];
+     active -> preboot [label = "recv(osdmap<down>)"];
+     active -> end [label = "kill(SIGINT)"];
+     active -> waiting_for_healthy [label = "i am unleathy!"]
+     prestop -> end [label = "recv(osdmap<down>)"];
+   }
+
+.. describe:: waiting_for_healthy
+
+   If an OSD daemon is able to connected to its heartbeat peers, and its own
+   internal hearbeat does not fail, it is considered healthy. Otherwise, it
+   puts itself in the state of `waiting_for_healthy`, and check its own
+   reachability and internal heartbeat periodically.
+
+.. describe:: preboot
+
+   OSD sends an `MOSDBoot` message to the connected monitor to inform the
+   cluster that it's ready to serve, so that the quorum can mark it `up`
+   in the osdmap.
+
+.. describe:: booting
+
+   Before being marked as `up`, an OSD has to stay in its `booting` state.
+
+.. describe:: active
+
+   Upon receiving an osdmap marking the OSD as `up`, it transits to `active`
+   state. After that, it is entitled to do its business. But the OSD service
+   can be fully stopped or suspended due to various reasons. For instance,
+   the osd services can be stopped by administrator manually, or marked `stop`
+   in the osdmap. Or any of its IP addresses does not match with the
+   corresponding one configured in osdmap, it transits to `preboot` if
+   it considers itself healthy.
+
+.. describe:: prestop
+
+   The OSD transits to `prestop` unconditionally upon request of `stop`.
+   But before bidding us farewell, it tries to get the acknowledge from
+   the monitor by sending an `MOSDMarkMeDown`, and waiting for an response
+   of updated osdmap or another `MOSDMarkMeDown` message.